INTRODUCTION AND LITERATURE REVIEW

The Connecting rods are widely used in variety of engines to transmit the thrust of the piston to the crankshaft, and results into conversion of the reciprocating motion of piston to the rotational motion of crankshaft. It consists of a pin-end, a shank section, and a crank-end. Pin-end and crank-end pin holes are machined to permit accurate fitting of bearings. One end of the connecting rod is connected to the piston with the help of a piston pin. The other end revolves with the crankshaft and is split to permit it to be clamped around the crankshaft. Depending upon the size of big end, the two parts are clamped by two or four bolts. Connecting rods are subjected to forces generated by mass and fuel combustion. These two forces results in axial and bending stresses. Bending stresses appear due to eccentricities, crankshaft, case wall deformation, and rotational mass force; therefore, a connecting rod must be capable of transmitting axial tension/compression and bending stresses caused by the thrust and pull on the piston and by the centrifugal force [1].

The connecting rods of the tractor are mostly made of cast iron through the forging or powder metallurgy. The main reason for applying these methods is to produce the components integrally and to reach high productivity with the lowest cost [2] and optimized shape [3]. The connecting rod design is complicated because the engine is to work in variably complicated conditions. The connecting rod is subjected to the varying pressure by the rod mechanism and inertia forces due to the acceleration/retardation in a cycle [4]. Biancolini et al. [5] carried out fatigue analysis and proposed the design of connecting rod. A rupture due to the fatigue and the method of correcting the connecting rod design parameters is reported by Rabb [6]. Beretta et al. [7] presented a strengthening method for the connecting rod design. Finite Element Method (FEM) is a modern technique for the fatigue analysis of connecting rod for the estimation of component longevity. FEM is capable of generating the stress/strain distributions throughout the component which enables us to find the critical points authentically. This method is extremely useful particularly when the component geometrical shape is complex and loading conditions are not sophisticated. In FEM, the influential component factors are able to change such as material, cross section conditions etc. and component optimization under the fatigue cyclic loading can be performed easily and quickly [6]. In Computer Aided Design, the analysis of a component is performed in a virtual environment without any necessity of making a prototype [8]; leading to savings in terms of time and cost.

For the reason that the connecting rod failure is usually due to the fatigue phenomenon, M. Omid et al. [9] performed FE analysis of U650 Tractor connecting rod on ANSYS software and concludes that, under reverse loading (tensile and compressive) the critical point is 46 (near the big end of the connecting rod). In order to improve on fatigue life of the connecting rod this value may be increased by decreasing the stress concentration coefficient.

In the present work, the modeling of connecting rod is done using Pro/E software and finite element fatigue analysis is carried out on ANSYS workbench. The aim is to investigate the effects of connecting rod design parameters to improve the performance under the reversible cyclic loadings. It is observed that the fatigue life can be improved by reducing the stress concentration coefficient by modifying some of the design parameters of connecting rod.

1.2 LITERATURE REVIEW

Despite the fact that most engineers and designers are aware of fatigue due to reversible cyclic loadings and a large amount of experimental data has been generated on the fatigue properties of various metallic and non-metallic materials, fatigue failures of engineering components are still common. The factors which influence the fatigue life of a component in service are

• complex stress cycles
• engineering design
• manufacturing and inspection
• service conditions and environment, and
• material of construction

The use of calculations and simulations is a key feature of the modern design process. Several properties such as stress, strength, stiffness, durability, handling, ride comfort and crash resistance, etc. can be numerically analyzed with varying levels of accuracy. Development time can be reduced by ensuring that some, or rather all, of these properties fulfil established requirements even before the first prototype is being built. Accordingly, calculations based on fatigue life and accurate loading histories permit structures and components to be optimized for durability without the need for expensive and time-consuming testing of series of prototypes. Thus, designs can be obtained that are less conservative (i.e., better optimized) than those based on traditional criteria, such as maximum load or stress for a series of standard load cases [15].

The use of Finite Element Method (FEM) for calculating stress and strain is a well established procedure in analyzing the fatigue and determining longevity of components. Del Llano-Vizcaya et al. [16] carried out finite element stress analysis in ANSYS code and performed multi-axial fatigue study of helical compression springs using the fatigue software. Biancolini et al. [5] designed a connecting rod based on fatigue analysis. Beretta et al. [7] presented a resistant method to failure on connecting rod design that improved the fatigue life slightly. They found that the occurrence of fatigue phenomenon is closely related to the appearance of cycling stresses within the connecting rod body.

Lu [17] presented an approach to optimize the shape of a connecting rod subjected to a load cycle which consisted of the inertia load deducted from gas load as one extreme and peak inertia load exerted by the piston assembly mass as the other extreme. A FE routine is first used for calculating the displacements and stresses in the connecting rod, which are further used in another routine to calculate the total life. Fatigue life is defined as the sum of crack initiation and crack growth lives, with crack growth life obtained using fracture mechanics. Rahman et al. [18] presented the finite element based fatigue life prediction of a new free piston linear generator engine mounting. The objective of investigations is to assess the critical fatigue locations due to loading conditions. They concluded that Morrow mean stress correction method gives the most conservative (less life) results for crack initiation method.

Nanaware and Pable [19] described a case study on the fatigue fracture of rear axle shafts of 575 DI tractors. The failure of rear axle shafts was due to inadequate spline root radius, which led to crack initiation and subsequent crack growth is by fatigue under the cyclic loading conditions of field operation. In general, the shafts in power plant systems run with a steady torsion combined with cyclic bending stress due to self-weight or weights of components or possible misalignment between journal bearings [20]. A similar case study [19] is reported in Fatigue Design Handbook AE 10 (Fatigue design handbook, 1988). The rear axle shaft failure of a scraper type tractor was considered as a case study. The rear axle shafts were failing within six months of service, even though durability tests were done in the laboratory. It was concluded that failure of shaft was due to the reverse torque.

Sarihan and Song [21] used a fatigue load cycle, consisting of compressive gas load corresponding to the maximum torque and tensile load corresponding to the maximum inertia load for the optimization of wrist pin end. They used the maximum loads for the entire operating range of the engine. Modified Goodman equation with alternating octahedral shear stress and mean octahedral shear stress are used for the fatigue design. They generated an approximate design surface and optimized the same. The objective and constraint functions are updated to obtain precise values. This process is repeated till the